1. Field of the Invention
The present invention relates to a three-phase DC motor.
2. Description of the Background Art
Conventionally, as disclosed in U.S. Patent Application Laid-Open No. 2006/0093694, motor-driven actuators are known. As shown in FIG. 7, the actuator disclosed in U.S. Patent Application Laid-Open No. 2006/0093694 has a ball-screw 63 horizontally mounted under a motor 61 with a drive shaft 61 a thereof laid horizontally. The drive shaft 61 a of the motor 61 and the ball-screw 63 are coupled to each other through a speed reduction mechanism 65. The ball-screw 63 rotates at a speed reduced at a predetermined rate relative to the rotation speed of the motor 61. Thus, the ball-screw 63 drives a driven unit or units. When the actuator is the one for driving a flight control surface of an aircraft, the driven unit is the flight control surface or a mechanism connected to the flight control surface.
Three-phase DC motors with an inner rotor is known as motors for driving an actuator, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2010-142006. This three-phase DC motor has a rotor that is free to spin inside a stator's cylindrical hollow. The stator has inwardly projecting teeth spaced equally in the circumferential direction. A coil is wound around each of these core units (teeth). These coils are, U-phase coils, V-phase coils, and W-phase coils which are connected in series.
When the three-phase DC motor with an inner rotor as disclosed in Japanese Patent Application Laid-Open No. 2010-142006 is used for the actuator for driving a flight control surface of an aircraft, there is a limit to reduce the thickness of the wing in which the actuator is housed. More specifically, the stator of the motor has a cylindrical shape, so that the wing is required to have a thickness sufficient for the diameter of the stator to house the actuator. However, stators with a smaller diameter obviously produce smaller motor torque. In order to keep desired motor torque, reduction in thickness of the wing in which the actuator is housed has a limitation. One measure to address this problem is to use a number of small power motors and combine the outputs of these motors to achieve a desired output. This configuration requires to combine torque with some difficulties in mechanical and control considerations.